1) Field of the Invention
The present invention relates to a color signal value extracting method and apparatus for extracting color signal values (for example, RGB values or L*a*b* values) from an image (for example, an RGB image or an L*a*b* image) of a patch (color region) on a color chart when a color transformation table for a color input device such as a color scanner or the like is created, gradation maintainability of a color input device such as a color scanner or the like is checked, or a color transformation table for a color output device such as a color printer or the like is created, a color transformation table creating method and apparatus applied the color signal value extracting method and apparatus, a gradation maintainability checking method and apparatus, and a record medium in which programs therefor are recorded.
2) Description of the Related Art
A color transformation table is generally used to match color appearance between different color input/output devices when color signals are transformed into color signals in a different color space.
Here, the color space signifies a coordinate system expressing colors. There are various color spaces. A color printer (color output device), a color scanner (color input device) and the like have a color space (device-dependent color space) depending on a device such as CMY (cyan, magenta, yellow) or RGB (red, green, blue). In a color printer whose maximum value and a minimum value of color signal values are 0 and 255, for example, output colors thereof are expressed by values in each of CMY within a range from 0 to 255. L*a*b* or XYZ which are color spaces not depending on a device are also widely used other than the above device-dependent color space. The color space not depending on a device can express an absolute color space.
Color transformation is to transform a color signal value expressed in a certain color space into a color signal value expressed in a different color space. When such color transformation is performed between color spaces of different devices, the above color space (L*a*b*, XYZ or the like) not depending on a device is used as a color space intermediate between the above device-dependent color spaces. For example, when color appearance of a color original is matched with color appearance of a printed matter that is obtained by out putting, by a color printer, an RGB image obtained by reading the color original by a color scanner, color transformation is performed in the following procedures (1) and (2):
(1) The RGB image obtained by a color scanner is color-transformed into an image expressed in, for example, the L*a*b* system.
(2) The L*a*b* image obtained in (1) is transformed into CMY data for a color printer, and outputted by the color printer on the basis of the CMY data.
In the color transformation described in the above procedures (1) and (2), a color transformation table (profile) is used. In the color transformation table, relationship between different color spaces is registered. In color transformation, the relationship is used. When a color registered in the color transformation table, color transformation is performed in such a manner that the color transformation table is looked up to read relationship of the color registered in the color transformation table. When a color not registered in the color transformation table is transformed, an interpolation process is performed on the basis of data registered in the color transformation table to transform a desired color.
When a color transformation table for a color input device used in the above procedure (1) is created, a color chart that is an original in which a number of colors are uniformly shown is generally used. An a practical example of the color chart, there is an input color target (ANSI/IT8.7/2) according to ANSI (American National Standards Institute) standard.
An input color target shown in FIG. 29 has a color patch region including 264 color patches and a gray patch region (gray scale) including gray patches in different 24 gradation levels.
In more detail, the color patch region is a part used when a color transformation table is created, which is a matrix-like region of 12 rows (A-L) by 22 columns (1-22), formed with 264 rectangular patches (square color regions). In the color patch region, 144 colors in a region of the rows A-B by the columns 1-12 are color solid colors, 84 colors in a region of the rows A-L and the columns 13-19 are primary color scales of C, M, Y, K, R, G, and B, and 36 colors in the region of the rows A-L and the columns 20-22 are maker's original colors.
The gray patch region is a part used when gradation maintainability of the color input device is checked, formed with 24 continuous rectangular patches. A gradation level of each rectangular patch is set in such a manner as to change from a lower level to a higher level (or a higher level to a lower level) when seen from left to right.
When a color transformation table for a color input device is created, color patch groups in the above input color target are read by a color input device, then color signal values (RGB values) of each color patch are extracted from an RGB image of the color patch groups obtained by the color input device, in general. Measured values (L*a*b* values) of each color patch in the input color target are separately obtained. Then, relationship between color signal values and measured values is stored as a look-up table, whereby a color transformation table for the color input device is created.
When the gradation maintainability that is one of characteristics of the color input device is grasped and checked, a gray scale (gradation original) of the above input color target is read by a color input device, and color signal values (RGB values) of each gray patch are extracted from an RGB image of the gray scale obtained by the color input device, in general. Then, whether the color signal values change in order of the gradation level of the gray patches or not, that is, whether inversion of the color signal values generates or not, is examined.
When a color transformation table for a color input device is created or when gradation maintainability of a color input device is checked as above, a process of extracting color signal values from a patch image ready by the color input device needs to be frequently performed.
As a means for extracting color signal values, there is a technique disclosed in Japanese Patent Laid-Open Publication No. 5-223642, for example. The technique disclosed in the above publication is aimed to improve colorimetric accuracy when colorimetric values (L*a*b* values, XYZ values) of an object are measured, in which a color signal extracting means is disclosed. There is a calorimeter of a non-contact type used to measure colors of an object of colorimetry. Use of such calorimeter may introduce a danger of decreasing the colorimetric accuracy since reflected light from an object other than the object of colorimetry is contaminated. To solve this drawback, the technique disclosed in the above publication provides the following procedures (a) to (c):
(a) beforehand measuring colors of each patch in the standard color chart, and holding results of the measurement as RGB data;
(b) photographing an object of colorimetry and the standard color chart simultaneously or separately by a color input device, designating each patch (six colors in black, gray, white, blue, green and red) of the standard color chart in the obtained image on a screen of a display by a cursor, and extracting RGB data (color signal values) of the designated patch; and
(c) obtaining a difference between the RGB data obtained beforehand in (a) and RGB data extracted from the standard color chart in (b), and compensating the object of colorimetry.
However, it is necessary to designate several tens to several hundreds of colors when a color transformation table is created or gradation maintainability is checked. Therefore, the color signal value extracting method (method in which a patch is designated by a cursor to extract color signal values) disclosed in the above publication requires a lot of time and labor to extract color signals from a number of color patch groups.
There is software for creating a color transformation table for a color input device (for example, ScanOpenICC by Hidelberg) commercially available as a means for creating a color transformation table for a color input device. In which, the user selects only four points in the corners of a color chart (input color target) in an image of the color chart read by a color input device, whereby color signal values of a number of patch groups are automatically obtained. The procedure for obtaining the color signal values are as follows:
(i) The user reads a color chart (for example, the input color target shown in FIG. 29) by a color input device. A color chart image obtained by reading is assumed to be accurately erected and free from any inclination.
(ii) The user designates four points in the corners (for example, C1, C2, C3 and C4 in FIG. 29) of the whole patch groups (color patches in 264 colors, or color patches and gray patches in 288 colors) in the color chart image to obtain coordinates of the image.
(iii) On the basis of the coordinates of the image in (ii), the software automatically obtains color signal values of each patch. Since a state of arrangement of the patch groups in the color chart is standardized, thus known, a position of each patch can be grasped on the basis of the known state of arrangement and the obtained coordinates of the image, and color signal values can be extracted and obtained from each patch on the basis of a result of the grasp.
When a color transformation table for a color output device used in the above procedure (2) is created, color signal values (RGB values or CMY values) are inputted to the color output device to output color patches having multicolor color patches corresponding to the color signal values from the color output device. Each color patch in the color chart outputted from the output device is measured by a colorimeter to obtain colorimetric values (L*a*b* values) of the color patch. Relationship between the color signal values inputted to the color output device with the colorimetric values obtained from each color patch is stored as a look up table, whereby a color transformation table for the color output device is created.
When a color transformation table for a color input device is created in the known color signal value extracting method (for example, the technique disclosed in the above publication), the following disadvantages arise.
(1) Since a large amount of relationship is required to create a color transformation table, it is necessary to extract a number of color signal values of an image obtained by reading patch groups in a color chart by a color input device and outputting, which leads to a considerable time and labor.
(2) When the number of extracted color signal values is large, there is a high possibility that the color signal values are correlated with colorimetric values in a wrong order.
(3) Even if patches in the same color are read, color signal values outputted from the color input device are quite different from pixel to pixel since the color signal values have been affected by characteristics of the device. For this, if color signal values of only one pixel is employed as values corresponding to colorimetric values, there is a danger of degradation of color transformation accuracy.
(4) Color signal values extracted from a region neighboring a patch in a different color are optically affected by the neighboring patch in a different color even if they are in a patch region in the same color. For this, if the color signal values are employed as values corresponding to colorimetric values, there is a danger of degradation of the color transformation accuracy.
Similarly, when gradation maintainability of a color input device is checked in the known color signal extracting method (for example, the method disclosed in the above publication), the following drawbacks arise.
(1) To grasp gradation maintainability, color signal values need to be extracted from a number of patches, which leads to a considerable time and labor.
(2) Even when patches in the same color are read, color signal values outputted from the color input device are quite different from pixel to pixel due to an effect of characteristics of the device. If evaluation is made using color signal values of only one pixel, there is a danger of degradation of evaluation accuracy of the gradation maintainability.
(3) Color signal values extracted from a region neighboring a patch in a different color are optically affected by the neighboring patch in a different color even in patch regions in the same color. If evaluation is made using such the color signal values, there is a danger of degradation of evaluation accuracy of the gradation maintainability.
In the above known software for creating a color transformation table for a color input device, the user recognizes a position of each patch by designating four points in the corners of a color chart image to automatically extract color signal values of each patch, based on that a relative positional relation of patch groups is known. This allows a color transformation table to be created readily.
However, the above software cannot check correctness of color signal values automatically extracted. The color transformation table is created without checking its correctness.
Particularly, if a position of a color chart is not strictly set when the color chart (for example, the input color target as shown in FIG. 29) is read by a color input device, the software might extract color signal values without accurately obtaining coordinates of the image of the patch groups. When the color chart is arranged on even small inclination, there is a possibility that a region of each patch from which color signal values are to be extracted by the software gets closer to a neighboring patch, which leads to noticeable degradation of accuracy of the color signal values. The known software has no means or measure to confirm a region from which color signal values have been extracted, thus a color transformation table is created without confirming the region.
Use of such the software for creating a color transformation table for a color input device allows easy extraction of color signal values and creation of a color transformation table. However, the software cannot check whether the color signal values are accurately extracted, so that accuracy or reliability of the created color transformation table (accuracy of color transformation) might degrade. Similarly, even if color signal values are extracted from a gray scale of the color chart using the software and the gradation maintainability is checked, it is impossible to check whether the color signal values have been accurately obtained or not although extraction of the color signal values is simple. This leads to degradation of the accuracy of such checking (evaluation accuracy).
Alternatively, it is possible to strictly set a position of a color chart when the color chart is read. In such case, setting of a position of the color chart is troublesome, which obstructs easy creation of a color transformation table or easy checking of gradation maintainability.
On the other hand, in the known method for creating a color transformation table for a color output device, it is necessary to measure patches in several hundreds of colors in a color chart outputted from a color output device. However, measuring several hundreds of colors requires a considerable time and labor, which requires a lot of time and labor to create a color transformation table for the color output device, as a result.